This invention relates to optical waveguide couplers and in particular to wavelength selective optical waveguide couplers.
It is known that, if two single mode optical waveguides dimensioned to provide matching propagation constants are arranged to extend alongside each other in sufficiently close proximity for their evanescent fields to overlap to an appreciable extent, then optical power will be coupled between the two waveguides. Power launched into one of the waveguides will, after a certain distance, be transferred in a single transfer substantially completely to the other. At twice this distance, the power is transferred back to the first waveguide. At three times this distance the power has been transferred for a second time to the second waveguide, and so on. The distance required for a single transfer of power from one waveguide to another is not completely independent of wavelength, but is such a weak function of wavelength as to make the resulting coupler substantially wavelength independent for most practical purposes in optical telecommunications application.